The present invention relates to a gate valve having an attachment which increases the force of closing as the gate approaches the final stages of its closing movement. U.S. Pat. No. 4,519,575 discloses a gate valve having an actuator which utilizes cam surfaces which are engaged by spring loaded cam following carriages and the cam surface is shaped to provide a larger force with the final closing movement of the valve so that it can shear a wire line extending therethrough. The closing force is transmitted from the following carriages to a plate secured to the valve stem. This valve relies entirely on the force from the cam follower for its closing force.
U.S. Pat. Nos. 4,445,424 and 4,523,516 disclose bellows type or Belleville washer type of springs used in actuators for valves.
U.S. Pat. No. 4,651,970 discloses a gate valve suitable for cutting wire line in its closing movement which includes an actuator and an attached energy storage assembly which is compressed in the initial stages of the actuator stroke to open the valve and releases the energy in the final stages of the valve closing stroke. This supplemental closing force is achieved by releasable latch mechanisms which release only near the end of the closing stroke so that the force of the compressed spring of the assembly is added to the actuator's closing force.